1. Field of the Invention
The present invention relates to a plasma CVD (Chemical Vapor Deposition) method for forming a thin film, and particularly to a method for forming a low dielectric constant insulation film for a semiconductor device.
2. Description of the Related Art
A plasma CVD film-forming method is a technique of forming a thin film on a substrate in a reaction space by generating a plasma by bringing microwaves or RF radio-frequency electric power into a reaction chamber. For methods of bringing electric power in, there are the capacity coupling method, the inductive coupling method, the electromagnetic wave coupling method and others. FIG. 1 shows an embodiment of plasma CVD equipment of a parallel-flat-plate type using a capacity coupling method. By placing two pairs of electrically conductive flat electrodes 101, 102 parallel to and opposing each other within a reaction chamber 104, applying RF power 105 to one side and grounding the other side, the plasma is excited between these two electrodes to form a film on a substrate 103. Radio-frequency electric power in a megahertz band of 13.56 MHz or 27 MHz or in a kilohertz band of 400 kHz is applied independently or by synthesizing them. In addition to this, there are the ICP method, the ECR method using microwaves, helicon wave plasma, and surface wave plasma, etc. In such film-forming equipment, a method in which a plasma source is placed at the top and a substrate on which a film is formed is placed at a lower stage, and the lower stage is electrically grounded or a bias voltage is applied, is widely used.
In conventional plasma CVD (chemical vapor deposition), a thin film is formed on a substrate by bringing a reaction gas in a deposition space in which the substrate, on which a thin film is formed, is placed and by making the reaction gas react by thermal and plasma energy. Raising the temperature of the reaction space and the intensity of plasma improves reactivity in the vapor phase, accelerating decomposition and polymerization of material gases.
Problems to be Resolved
With a conventional reactor structure, however, because a substrate is exposed to the reaction space, the temperature of and near a substrate surface rises if the temperature of the reaction space and the intensity of plasma are raised. Consequently, the intensity of plasma irradiated on the substrate surface is raised, advancing the reaction of products absorbed and deposited on the substrate surface. It is possible to form products with various molecular weights in a vapor-phase reaction by selecting the type of a reaction gas or adjusting the reactivity of the reaction gas. Particularly to deposit a high vapor pressure product with a relatively low molecular weight on a wafer substrate, it is necessary to lower the temperature of and near the substrate surface. To accelerate the reactivity in the vapor phase, it is necessary to raise the temperature of the reaction space and the intensity of plasma. This, however, raises the temperature of a wafer substrate as well and makes it difficult to deposit high vapor pressure products. When depositing reaction products in an interim phase, a problem occurs wherein the reaction of the product deposited on the substrate advances by plasma irradiation.
To avoid this problem, lowering the temperature of and near the substrate surface and the intensity of plasma irradiated on the substrate by lowering the temperature of the reaction space and the intensity of plasma is considered. By doing this, it becomes possible to deposit a high vapor pressure product and slow down a reaction occurring on the substrate surface become possible. In this case, however, reaction efficiency of the reaction space decreases. Using conventional equipment, the reactivity of the reaction space cannot be increased, the temperature of a substrate cannot be lowered, and a reaction occurring on the substrate surface cannot be suppressed.
In an embodiment of the present invention, provided is a CVD apparatus for forming a thin film on a semiconductor substrate by plasma reaction, comprising: (i) a reaction chamber; (ii) a reaction gas inlet for introducing a reaction gas into the reaction chamber; (iii) a lower stage on which a semiconductor substrate is placed in the reaction chamber, said lower stage functioning as a lower electrode; (iv) an upper electrode for plasma excitation in the reaction chamber; (v) an intermediate electrode with plural pores through which the reaction gas passes, said intermediate electrode being disposed below the upper electrode, wherein a reaction space is formed between the upper electrode and the intermediate electrode; and (vi) a cooling plate with plural pores through which the reaction gas passes, said cooling plate being disposed between the intermediate electrode and the lower stage, said cooling plate being controlled at a temperature lower than the intermediate electrode, wherein a transition space is formed between the intermediate electrode and the cooling plate, and a plasma-free space is formed between the cooling plate and the lower stage.
In another aspect of the present invention, provided is a method for forming a thin film on a semiconductor substrate by plasma reaction, comprising the steps of: (a) introducing a reaction gas into an upper section of a reaction chamber; (b) exciting a plasma in the upper section to react the reaction gas by applying electrical power between an upper electrode and a lower stage on which a substrate is place; (c) enclosing the plasma in the upper section by providing below the upper electrode an intermediate electrode having the same electrical potential as the lower stage, said intermediate electrode having plural pores through which the activated reaction gas passes; (d) cooling a section under the upper section by controlling a cooling plate disposed between the intermediate electrode and the lower stage at a temperature lower than the intermediate electrode, said cooling plate having plural pores through which the cooled reaction gas passes, wherein a middle section is formed between the intermediate electrode and the cooling plate; and (e) controlling the lower stage at a temperature lower than the cooling plate, wherein a lower section is formed between the cooling plate and the lower stage, whereby reaction products accumulate on the substrate.
General Aspect of the Present Invention
The present invention can generally be characterized in that it has a reactor structure, in which a reaction space and a deposition space are separated by an intermediate electrode kept at a high temperature and a low-temperature plate whose temperature is adjusted at a low temperature, and which realizes two completely different environments, a reaction space (a Hot area) which is in a high reactivity state at a high temperature and in a plasma state, and a deposition space (a Cool area) which is at a low temperature and substantially in a non-plasma state.
This reactor enables deposition of a high vapor pressure product with a low molecular weight while realizing high reaction efficiency. In a high-reactivity space (Hot area) where a plasma is excited at a high temperature, by decomposing or polymerizing material gases by thermal and plasma energy, the material gases can effectively react in the vapor phase. By passing a reaction gas through the low-temperature Cool area in which substantially no plasma exists, the reaction can be terminated in the vapor phase and simultaneously gases generated are condensed, and high vapor pressure molecules containing water and alcohol can be liquefied and deposited on a substrate.
For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
Further aspects, features and advantages of this invention will become apparent from the detailed description of the preferred embodiments which follow.